Abstract
The reaction rates of the reactions H- + H lrarr H2 + e, H + e → H- + hv, H- + e → H + 2e and H2 + H → 3H are given as functions of temperature. Those rates are obtained from the best data available and are used to study the chemical evolution of a transparent cloud of neutral hydrogen for temperatures T 8 0001 K. For temperatures T ≥ 3000 °K H2 production is never significant. For temperatures T ≤ 2000°K practically all the initial hydrogen bounds into H2 with a scaletime τ = 1/2a(He) N(e0). If a (H2H) is actually overestimated (and T ≤ 8000°K) then the initial hydrogen always bounds into molecules with the scaletime r.
